AFAIK, SYNC connects to *proprietary* closed-network mobile wireless via satellite--it is NOT a cellular ISP like your smartphone but rather more like a standalone GPS unit or satellite radio. (Analogously, you can't get "wi-fi" or mobile broadband through your Garmin or an XM receiver). You can plug a USB wireless network modem into your 3G-equipped web-enabled mobile device, which you could then plug into your car's USB port and stream internet audio---but ONLY if you have a contract for mobile broadband with the maker of that USB modem AND you are in the coverage area. A regular Linksys-type USB wi-fi (as opposed to moblie broadband) is useless on the go, unless you actually are within range of an OPEN conventional wi-fi network (or one for which you have the login info).
If you want to give the kids internet access, buy a cellular carrier's mobile-broadband hotspot or USB modem for their laptop, or tether an internet-enabled phone to it. But be prepared to lose signal as you drive out of the coverage area.